This is a known problem in Windows 10 mobile.
It's possible to tap on the screen when an opened e-mail renders incorrectly.
This will tighten the viewed screen somewhat yet not completely.
I usually try to pinch in the screen so the content won't be larger than the screen.
Also holding the phone in landscape may force a proper resize.
This is very annoying and we're hoping for the best as far as a fix.
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